Autophagy is a cellular degradation process that involves the delivery of cytoplasmic cargo such as long-lived protein, mis-folded protein or damaged organelles, sequestered inside double-membrane vesicles (autophagosome) before entering lysosome for degradation. Autophagy occurs at low basal levels in cells to maintain normal homeostatic functions by turnover of proteins and organelles. Upon cellular stressful conditions such as nutrient deprivation, oxidative stress, infection or protein aggregate accumulation, autophagy starts with membrane isolation and expansion to form autophagosome that sequesters all unwanted cytoplasmic materials. Followed by fusion of the autophagosome with lysosome to form an autolysosome, all the engulfed materials are degraded to recycle intracellular nutrients and energy [1]. Both autophagy impairment and the age-related decline of autophagic function lead to the pathogenesis of many age-related diseases such as neurodegenerative disorders and cancers [2].
One of the key roles for autophagy is to degrade toxic aggregate-prone cytoplasmic proteins that are inaccessible to the proteasome when they form oligomers or aggregates [3]; aggregate-prone proteins with polyglutamine and polyalanine expansions, in turn, are degraded by autophagy [4]. Inhibition of mTOR induces autophagy and reduces toxicity of polyglutamine expansions or mutant proteins in fly and mouse models [4-7]. These mutant proteins include mutant α-synuclein which causes Parkinson's disease, and polyglutamine-expanded mutant huntingtin that causes Huntington's disease [8,9]. In contrast, protein aggregates form in the cytoplasm when autophagy is inhibited in normal mice [10]. Rapamycin, a United States Food and Drug Administration (FDA)-approved immunosuppressant, is found effective in treating fruit fly and mouse models of Huntington's disease through increased autophagic clearance of mutant huntingtin [5]. Besides, a small-molecule screen also revealed new chemicals that attenuate the toxicity of mutant huntingtin through autophagy [9].
While autophagy may play a protective role in neurodegenerative diseases [9], autophagic dysfunction is associated with DNA damage, chromosome instability [11, 12], and increased incidence of malignancies [12]. Modulators of autophagy may play a protective role through promoting autophagic cell death in tumors or augmenting the efficacy of chemotherapeutic agents when used in combination. Several clinically approved or experimental antitumor agents induced autophagy-related cell death in various types of cancer cells [13-16].
Recently, natural compounds from alkaloids have been found to induce autophagy with potential neuroprotective or anti-cancer effects. For instance, alkaloids isolated from Chinese herbal medicine are important source for drug discovery [17]. Alkaloids such as berberine, matrine and tetrandrine, exhibit their anti-cancer effects through cell cycle arrest, apoptosis, autophagy, inhibition of metastasis or angiogenesis [18-20]. Camptothecin and vinblastine are chemotherapeutic drugs that have been approved for clinical use [21,22]. In addition, alkaloids such as isorhynchophylline [23] and berberine [24] were also reported for their neuroprotective effects in vitro.